Polyethylene compositions having improved optical and mechanical properties and improved processability in the melted state

ABSTRACT

Polyethylene composition comprising (percentages by weight): A) from 60 to 95% of an LLDPE copolymer having a density from 0.905 to 0.9 g/cm 3 , M w /M n  values less than 4 and F/E ratio values greater than 20; B) from 5 to 40% of one or more crystalline copolymers of propylene selected from among defined copolymers of propylene with ethylene and/or higher alpha-olefins.

The present invention concerns polyethylene compositions comprising alinear low-density copolymer of ethylene (LLDPE) having a narrowdistribution of molecular weights, and a crystalline copolymer ofpropylene.

The films obtainable from the said compositions possess an excellentbalance of mechanical properties and optical properties.

Moreover, the aforesaid compositions are readily processable in themelted state, since they do not require large expenditures of energy inthe machines used for their processing and do not cause high pressuresat the head in the machines themselves.

The copolymer of ethylene used for the compositions of the presentinvention possesses a molecular weight distribution, in terms of theratio between the average ponderal molecular weight (M_(w)) and theaverage numerical molecular weight (M_(n)), that is in terms ofM_(w)/M_(n), which is particularly narrow (corresponding to values ofM_(w)/M_(n) less than 4) and hence typical of the polyethylenes obtainedwith metallocene catalysts.

In that respect, the compositions of the present invention differ fromthe compositions described in the published patent applications WO93/03078 and WO 95/20009, in which the LLDPE copolymer (which is mixedwith a crystalline copolymer of propylene) is prepared withZiegler-Natta catalysts and hence possesses values of M_(w)/M_(N)typically greater than or equal to 4.

According to the U.S. Pat. No. 4,871,813, it is possible to prepareLLDPE copolymers having M_(w)/M_(n) values less than 4 (from 2.5 to 6)even when the catalyst used is of the Ziegler-Natta type, however in theexamples only an LLDPE copolymer having a M_(w)/M_(n) value of 4 isused.

Also in the cited US patent the LLDPE copolymer is mixed with acopolymer of propylene, however the crystallinity of that copolymer ofpropylene is rather low, as is indicated by the low values of theenthalpy of fusion (less than or equal to 75 J/g) and, in particular,the degree of crystallinity (less than 35%).

The aforesaid documents show that by the addition of the copolymer ofpropylene to the LLDPE copolymer, polyethylene compositions havingimproved processability in the melted state, in the aforesaid sense, areobtained.

According to the U.S. Pat. No. 4,871,813, this effect is obtainedwithout substantially changing the optical and mechanical properties ofthe film, compared to those of a film obtained from the pure LLDPEcopolymer.

According to the published patent application WO 95/20009, as well asimproving the processability in the melted state, the addition of thecrystalline copolymer of propylene is capable of improving theresistance of the polyethylene film to impact and to tearing.

However, the optical properties of the compositions comprising an LLDPEcopolymer obtained with Ziegler-Natta catalysts and a copolymer ofpropylene are inferior, in particular as regards the haze and glossvalues, to those typical of an LLDPE copolymer obtained with metallocenecatalysts and having relatively high F/E ratio values, i.e. greater than20.

The aforesaid LLDPE copolymers obtained with metallocene catalysts ingeneral show haze values less than ca. 20% and gloss values greater thanca. 30% (measured on blown film of thickness 25 μm by the methoddescribed in the examples).

Corresponding to the said high haze and gloss values, such LLDPEcopolymers also possess satisfactory impact resistance (Dart Test) andtear resistance (Elmendorf) values.

However, the LLDPE copolymers obtained with metallocene catalystsdisplay unsatisfactory processability in the melted state.

Hence, it would be particularly desirable to obtain polyolefincompositions having the aforesaid optical properties, with the bestbalance possible of impact resistance and tear resistance and goodprocessability in the melted state.

In the U.S. Pat. No. 5,674,945, polyethylene compositions comprising anLLDPE copolymer obtained with metallocene catalysts and a copolymer ofpropylene having a density greater than or equal to 0.900 g/cm₃ aredescribed.

In particular, in the examples a copolymer containing 0.2 mol % ofbutene and a copolymer containing 3.4 mol % of ethylene and 1.6 mol % ofbutene are used.

In both cases, the relative quantity of propylene copolymer in thepolyethylene compositions is 10% by weight, and the transparency of thefilms obtained from such compositions is found to be substantiallyunchanged compared to the transparency of the films obtained from thecorresponding LLDPE copolymers in the pure state.

Moreover, the films obtained from the aforesaid compositions show hightensile modulus values, superior to those of the films obtained from thecorresponding LLDPE copolymers in the pure state, and high breakingstrain values.

The technical problem consisting in the obtention of excellent balancesof optical properties, impact resistance and tear resistance is notconsidered.

Polyethylene compositions have now been created which fully satisfy theaforesaid requirements, thanks to an unusual and particularly favourablebalance of mechanical and optical properties and processability in themelted state.

Hence the object of the present invention is constituted by polyethylenecompositions comprising (percentages by weight):

A) from 60 to 95%, preferably from 60 to 90%, more preferably from 70 to88%, of a copolymer of ethylene with an alpha-olefin CH₂═CHR, in which Ris an alkyl radical containing from 1 to 18 atoms of carbon (LLDPEcopolymer), the said copolymer having a density from 0.905 to 0.935g/cm³, preferably from 0.910 to 0.930, more preferably from 0.915 to0.925 g/cm³ (measured according to ASTM D 4883), M_(w)/M_(n) values lessthan 4, preferably from 1.5 to 3.5, more preferably from 1.5 to 3(measured by GPC, i.e. gel permeation chromatography) and F/E ratiovalues greater than 20, preferably from 25 to 70, more preferably from25 to 50 (measured according to ASTM D 1238);

B) from 5 to 40%, preferably from 10 to 35%, more preferably from 12 to30%, of one or more crystalline copolymers of propylene selected fromamong (i) copolymers of propylene with ethylene containing from 3 to 8%,preferably from 4 to 6%, of ethylene; (ii) copolymers of propylene withone or more alpha-olefins CH₂═CHR^(I), where R^(I) is an alkyl radicalhaving from 2 to 8 carbon atoms or an aryl radical, containing from 6 to25%, preferably from 8 to 20%, of alpha-olefins CH₂═CHR^(I); (iii)copolymers of propylene with ethylene and one or more alpha-olefinsCH₂═CHR^(I), where R^(I) has the aforesaid meaning, containing from 0.1to 8%, preferably from 0.5 to 5%, more preferably from 1 to 4%, ofethylene, and from 0.1 to 20%, preferably from 1 to 15%, more preferablyfrom 2.5 to 15%, in particular from 2.5 to 10%, of alpha-olefinsCH₂═CHR^(I), on condition that the total content of ethylene andalpha-olefins CH₂═CHR^(I) in the copolymers (iii) is greater than orequal to 5%.

Optionally, in order to improve the optical properties, the compositionsof the present invention can contain, in addition to the components A)and B), from 0.5 to 10%, preferably from 1 to 6%, by weight of an LDPEpolyethylene (component C), relative to the total weight of A) +B) +C).

As seems clear from the foregoing description, polymers containing twoor more types of comonomers are also included in the definition ofcopolymers.

The aforesaid compositions are generally characterized by haze valuesless than or equal to 25%, preferably less than or equal to 20%, inparticular between 20 and 5%, and gloss values greater than ca. 30%, inparticular between 30 and 60% (measured on blown film of thickness 25 μmby the method described in the examples).

Moreover, the compositions of the present invention are generallycharacterized by dart test values greater than or equal to 150 g,preferably greater than or equal to 200 g, in particular between 150 and400 g, preferably between 200 and 400 g (measured on blown film ofthickness 25 μm by the method described in the examples).

Moreover, the compositions of the present invention show particularlyhigh tear resistance values (Elmendorf), both as such and inconsideration of the dart test values. Generally, such values aregreater than or equal to 400 g (measured on blown film of thickness 25μm by the method described in the examples) in the transverse direction(TD), in particular between 400 and 800 g, and greater than or equal to150 g in the machine direction (MD), preferably greater than or equal to200 g, in particular between 150 and 350 g, preferably between 200 and350 g.

Generally, the component A) of the compositions of the present inventionhas an ethylene content greater than or equal to 60% by weight, inparticular from 60 to 99%, preferably greater than or equal to 70% byweight, in particular from 70 to 99%, more preferably greater than orequal to 80% by weight, in particular from 80 to 99%.

Examples of alpha-olefins CH₂═CHR present in the component A) of thecompositions of the present invention are propylene, 1-butene,1-pentene, 1-hexene, 4-methyl-1-pentene, 1-octene, 1-decene, 1-dodecene,1-tetradecene, 1-hexadecene, 1-octadecene and 1-eicosene.

Preferred examples are 1-butene, 1-hexene and 1-octene.

Generally, the component A) possesses melt flow rate E values (MFR Eaccording to ASTM D 1238) from 0.1 to 100 g/10 min. Moreover, the saidcomponent A) preferably possesses a content of fraction soluble inxylene at 25° C. less than or equal to 5% by weight.

The DSC (Differential Scanning Calorimetry) trace of the said componentA) preferably shows a single melting peak (typical of one crystallinephase); this peak is generally situated at a temperature greater than orequal to 100°0 C.

The LLDPE copolymers having the characteristics quoted above for thecomponent A) are known in the art and can be obtained by conventionalpolymerization processes (in gas phase, in suspension or in solution)using catalysts comprising a compound of a transition element,preferably Ti, Zr or Hf, or from the lanthanide series, wherein thiselement is bound to at least one cyclopentadienyl group, and acocatalyst, in particular an alumoxane or a compound capable of formingan alkyl cation.

Examples of the aforesaid catalysts and polymerization processes aredescribed in the published patent application WO 93/08221.

Examples of alpha-olefins CH₂═CHR^(I) present in the component B) of thecompositions of the present invention are 1-butene, 1-pentene,4-methylpentene-1, 1-hexene and 1-octene. 1-butene is preferred.

Generally, the component B) has melt flow rate L values (MFR L accordingto ASTM D 1238) from 0.1 to 500 g/10 min, preferably from 1 to 50, morepreferably from 6 to 25 g/10 min.

Preferably the said component B) has density values (measured accordingto ASTM D 4883) less than 0.9 g/cm³, in particular from 0.890 to 0.899,more preferably from 0.892 to 0.899.

Moreover, the said component B) preferably possesses the followingcharacteristics:

content of fraction insoluble in xylene at 25° C. greater than 70% byweight, more preferably greater than or equal to 75%, in particulargreater than or equal to 85% by weight;

enthalpy of fusion (measured according to ASTM D 3418-82) greater than50 J/g, more preferably greater than or equal to 60 J/g, in particulargreater than or equal to 70 J/g, for example from 75 to 95 J/g;

melting point (measured according to ASTM D 3418-82) less than 140° C.,more preferably from 120 to 140° C.;

M_(w)/M_(n) values greater than 3.5, in particular from 3.5 to 15.

The crystalline copolymers of propylene having the characteristicsquoted above for the component B) are known in the art, and can beobtained by conventional polymerization processes using stereospecificZiegler-Natta catalysts supported on magnesium halides. Such catalystscontain, as an essential component, a solid catalytic componentcomprising a compound of titanium having at least one titanium-halogenbond and one electron-donor compound, both supported on a magnesiumhalide. As cocatalysts, an Al-alkyl compound and an electron-donorcompound are generally used.

Catalysts having the aforesaid characteristics are for example describedin the U.S. Pat. No. 4,399,054 and in the European patent 45977.

The LDPE polymer (low density polyethylene) constituting the componentC) of the compositions of the present invention is a homopolymer ofethylene or a copolymer of ethylene containing smaller quantities ofcomonomers, such as butyl acrylate, prepared by polymerization at highpressure using free radical initiators.

The density of the said LDPE polymer generally ranges from 0.910 to0.925 g/cm³ (measured according to ASTM D 4883).

The MFR E values of the said LDPE polymer generally range from 0.1 to 50g/10 min, preferably from 0.3 to 20 g/10 min.

The LDPE polymers having the characteristics quoted above for thecomponent C) are known in the art. Specific examples are thecommercially available polymers with the brand names Escorene, andLupolen (BASF).

In addition to the aforesaid components, the compositions of the presentinvention can contain other polymeric components, such as olefinelastomers, in particular ethylene/propylene (EPR) or ethylene/propylene/diene (EPDM) elastomers, and additives commonly used in theart, such as stabilizers (in particular phenolic antioxidants andprocess stabilizers such as the organic phosphites), pigments, fillers,nucleating agents, release agents, lubric-ating and antistatic agents,flame retardants and plasticizers.

The compositions of the present invention can be prepared by processesof polymerization in two or more consecutive stages, using in at leastone stage the catalysts described above for the preparation of thecomponent A) and in at least one other stage the Ziegler-Natta catalystsdescribed above for the preparation of the component B), and optionallyadding the component C) by mixing in the melted state.

Naturally, it is also possible to prepare the compositions of thepresent invention by mixing of the components A), B), and optionally C),in the melted state.

The processes of mixing in the melted state advantageously used are ofconventional type and are based on the use of mixing devices known inthe art, such as single-screw and double-screw extruders.

In view of their ready processability in the melted state and theirexcellent optical and mechanical properties, the compositions of thepresent invention are particularly suitable for the preparation ofmoulded articles in general, and in particular of film, monolayer ormultilayer, whether cast or mono- or biaxially oriented, including blownfilms, in which at least one layer comprises the aforesaid compositions.

The processes for the preparation of blown film are well known in theart and comprise a stage of extrusion through a head with an annularaperture.

The product from this stage is a tubular extrudate which is theninflated with air,- to obtain a tubular bubble which is cooled andcollapsed to obtain the film.

The following examples are given in order to illustrate but not to limitthe present invention.

For these examples, the following materials are used:

A) LIMPE Copolymer Ethylene/1-octene copolymer, marketed by DOW CHEMICALCOMPANY under the name “Affinity-1570”, containing 10.8% by weight ofI-octene (determined by ^(—)C NMR) and having the followingcharacteristics:

Density (ASTM D 4883): 0.9130 g/CM³

M_(w)/M_(n) (GPC): ca. 2.5

MFR E (ASTM D 1238): 1.0 g/l0 min

F/E (ASTM D 1238): 41

Soluble in xylene at 25° C. (% by wt): 3.2

B) Crystalline Propylene Copolymer

A propylene copolymer having the following characteristics is used:

Butene content (% wt): 5.3

Ethylene content (% wt): 2.2

Density (g/cm³): 0.895

MFR L (g/10 min): 6

Soluble in xylene at 250° C. (% wt): 10

Note: The aforesaid contents of butene and ethylene are measured by IRspectroscopy, the density by ASTM D 4883, and the content insoluble inxylene (and hence the content soluble) is determined by the followingmethod:

2.5 g of copolymer together with 250 cm³ of o-xylene are placed in aconical glass flask, equipped with condenser and magnetic stirrer. Thetemperature is raised until the boiling point of the solvent is reachedin 30 min. The clear solution thus formed is left at reflux withstirring for another 30 min. The closed flask is then placed in a bathof water and ice for 30 min. and then in a bath of water thermostattedat 25° C. for 30 min. The solid formed is filtered on paper at a highfiltration rate. 100 cm³ of the liquid obtained from the filtration arepoured into an aluminium container, previously weighed, and the whole isplaced on a heating plate to evaporate the liquid in a current ofnitrogen. The container is then placed in an oven at 80° C. and keptunder vacuum until constant weight is attained.

The aforesaid propylene copolymers are prepared using Ziegler-Nattacatalysts of high yield and stereospecificity, supported on magnesiumchloride, in the polymerization.

EXAMPLE 1

The aforesaid components A) and B) are mixed in the melted state in asingle-screw extruder (Bandera TR-60) under the following conditions:

Temperature profile: 185, 195, 200, 205, 210, 215, 235, melted 230° C.;

Screw revolutions: 70 rpm;

Throughput: 67 kg/hr.

The relative quantities of the aforesaid components are equal to 80% byweight of A) and 20% by weight of B), relative to the total weight ofthe composition.

From the composition thus obtained, a blown film of 25 μm thickness isprepared using a COLLIN-25 machine under the following conditions:

Temperature profile: 155, 165, 175, 185, 190, 190, 190, 190, melted 200°C.;

Screw revolutions: 90 rpm; Throughput: 4.2 kg/h;

Blow-up ratio: 2.5.

The properties shown in Table 1 are measured on the film thus prepared.For comparison purposes, Table 1 also shows the properties of a blownfilm obtained and subjected to testing under the same conditions as inExample 1, but using the component A) in the pure state (ReferenceExample 1).

TABLE 1 Ex. No. 1 Ref.1 Head pressure 15.9 18.8 (Mpa) Motor input (A)6.7 7.2 Haze (%) 11 8 Gloss (%) 49 56 Dart Test (g) 252 480 Elmendorf TD(g) 510 545 MD (g) 220 215

With reference to Table 1, the head pressure corresponds to the pressuremeasured at the head of the extruder while the motor input relates tothe extruder motor.

Further, the properties of the films shown in Table 1 are measured bythe following standard ASTM methods:

Haze: ASTM D 1003

Gloss: ASTM D 2457

Dart Test: ASTM D 1709

Elmendorf: ASTM D 1922.

What is claimed is:
 1. Polyethylene composition comprising (percentagesby weight): A) from 60 to 95% of a copolymer of ethylene with analpha-olefin CH₂═CHR, wherein R is an alkyl radical containing from 1 to18 atoms of carbon, the said copolymer having a density from 0.905 to0.935 g/cm³, M_(w)/M_(n) values less than 4, and F/E ratio valuesgreater than 20; B) from 5 to 40% of one or more crystalline copolymersof propylene selected from among (i) copolymers of propylene withethylene containing from 3 to 8% of ethylene; (ii) copolymers ofpropylene with one or more alpha-olefins CH₂═CHR^(I), where R^(I) is analkyl radical having from 2 to 8 carbon atoms or an aryl radical,containing from 6 to 25% of alpha-olefins CH₂═CHR^(I); (iii) copolymersof propylene with ethylene and one or more alpha-olefins CH₂═CHR^(I),where R^(I) has the aforesaid meaning, containing from 0.1 to 8% ofethylene and from 0.1 to 20% of alpha-olefins CH₂═CHR^(I), on conditionthat the total content of ethylene and alpha-olefins CH₂═CHR^(I) in thecopolymers (iii) is greater than or equal to 5%.
 2. The polyethylenecomposition of claim 1 containing, in addition: C) from 0.5 to 10% byweight, relative to the total weight of A)+B)+C), of LDPE polyethylene.3. The polyethylene composition of claim 1, wherein the component B) hasa density less than 0.900 g/cm^(3.)
 4. The polyethylene composition ofclaim 1, wherein the component B) has a fraction insoluble in xylene at25° C. greater than 70% by weight.
 5. The polyethylene composition ofclaim 1, having haze values, measured on blown film of thickness 25 μmaccording to ASTM D 1003, less than or equal to 25%.
 6. Mono- ormultilayer film, wherein at least one layer comprises the polyethylenecomposition of claim
 1. 7. Blown film according to claim 6.